This invention relates to the field of coatings, especially transparent coatings for clear plastic surfaces, such as those used in making intra-ocular lenses and contact lenses.
Transparent plastics and glass are normally hydrophobic and are not wetted uniformly by water, which forms beads and rivulets on the surface and distorts the optical transmission of the lens, prism, or pane. When warm, moisture-laden air impinges on the plastic or glass surface, a film of tiny water droplets condenses thereon and destroys the optical clarity of the transmitted light. The novel, durable coating of the present invention provides uniform light transmission without fogging and eliminates optical distortion in both the wet and dry state.
It is well known that certain water-soluble materials such as glycerine can be wiped or sprayed onto an automobile windshield to prevent beading by raindrops. Scuba divers and deep-sea divers are also familiar with the practice of rubbing sputum on the inner surface of the facepiece or lens in order to delay fogging of the inner surface. However, such measures are only temporary and are therefore unsatisfactory.
Silicone coatings have been claimed to provide more permanent protection against fogging, but side-by-side comparison of coated and uncoated panels shows that the improvement is less than desired. Conditions prescribed for the coating process by vendors are in some cases above the glass temperature of the plastic to be coated and therefore damaging to the optical device. Coatings offered by certain vendors produce devices with mediocre fogging resistance and surprisingly poor optical properties.
Hydrophilic coatings, such as those described in U.S. Pat. No. 4,663,233, for example, are durable and show non-fogging and non-beading behavior when wet, but tend to fog in a cold, dry atmosphere.
As will be shown, the durable coatings of this invention are non-fogging at elevated and low temperatures, and at high and low humidity. This novel and useful characteristic is retained under practical conditions of exposure not Just for minutes, but for long periods.